Incremental vacuum dehydration-decarbonation experiments on a natural gibbsite (α-Al(OH3)): CO2 abundance and δ13C values

Incremental vacuum dehydration-decarbonation experiments were performed at 190°C on chemically “cleaned” aliquots of a gibbsite-dominated, Eocene-age bauxite sample with evolution of CO2 and H2O. “Plateau” F (CO2/H2O ratios) and δ13C values of the CO2 derived from gibbsite were attained over the dehydration interval, Xv(H2) = 0.16 to 0.67 (i.e., 16 to 67% breakdown of gibbsite). The plateau value of F for gibbsite was 0.0043 ± 0.0003, while the corresponding δ13C value of evolved CO2 was −16.0‰±0.4‰. Additional experiments on chemically cleaned aliquots included (1) treatment with a solution of 0.3M Na-Citrate + 0.1M Na-Dithionite and (2) an exchange experiment with 0.1 bar of 13C-depleted CO2 (−46‰) at 105°C for 64.5 h. Neither of these additional treatments resulted in a measurable perturbation of plateau values of F or δ13C for CO2 evolved from gibbsite during dehydroxylation. These results support published work on Holocene samples which suggested that CO2 occluded in gibbsite may preserve information on δ13C values of CO2 in ancient terrestrial systems. The plateau values of F observed in the Eocene gibbsite indicate that it may be possible to experimentally calibrate a relationship between the concentration of CO2 occluded in gibbsite and CO2 in the environment at the time of crystallization. Such a calibration would significantly enhance the value of gibbsite as a source of information on ancient oxidized carbon systems.